The invention relates to monitoring devices and more particularly, to methods and apparatus for detecting malfunctions in the operation of fluid dispensers.
Typical fluid dispensing systems in one form include a pump having an inlet connected to a supply of material and a discharge connected to a fluid dispenser. For precision dispensing, the dispenser may include a valve which permits fluid to pass through a discharge opening such as a spray nozzle or fluid tip. In some systems, the dispenser valve is operated by a programmed control device so that fluid is dispensed in precise or metered amounts.
In many applications it is often desirable that precise patterns, metered amounts or both be dispensed. In operation, precision or accurate metering is affected by many factors including nozzle wear, fluid impurities, nozzle clogging, and pump performance. Clogging of the material flow path, especially in the dispenser, is a typical problem that adversely affects the performance of precision dispensing systems. For example, in precision dispensing systems used to coat the interior surface of multipiece can bodies, a clogged or worn spray nozzle may cause the can body to be incompletely or improperly coated.
The can bodies are typically coated during the manufacturing process at rates of up to several hundred cans per minute. Thus, an improperly functioning dispenser, and more particularly, a clogged or worn nozzle can result in many improperly coated cans before detection of the fluid dispenser malfunction. An improperly coated can may have an adverse effect on the can's ability to function for storage. In some cases, the can may suffer accelerated deterioration (i.e., shortened shelf life), and in others (e.g. for foods and beverages) the contents may be adversely affected (e.g., taste, spoilage). Improper coating, therefore, is undesirable and adds substantial expense because improperly coated cans must be rejected and disposed of, or reprocessed by inspecting, hand sorting, cleaning and recoating.
The above problems are addressed by the fluid dispenser monitor described in U.S. Pat. No. 4,668,948 issued on May 26, 1987 to S. L. Merkel which is assigned to the assignee of this invention. The monitor utilizes an analog control system in which a calibrated orifice is used to provide, during the gun ON time, a small pressure drop from the static pressure set by the operator. The pressure is measured between the nozzle and the calibrated orifice both during the gun ON and OFF times to monitor fluid flow conditions through the gun. During the ON time, the pressure drop across the orifice may, for example, be approximately 50-60 pounds per square inch ("psi") given a static pressure of, for example, 800 psi. As the gun is turned ON and OFF to coat each successive can, the magnitude of the firing pressure is compared to a reference signal to detect adverse flow and pressure conditions. A counter is used to sense a predetermined number of firing pressure fault conditions before an error signal is generated.
The control system is operative during the coating process to create a alarm error signals if the firing pressure detected by the pressure transducer is greater than predetermined high or low pressure reference signals. Adverse flow conditions may result from worn or clogged nozzles; and when the detected pressure signal exceeds the pressure reference signal, alarm signals are generated to the operator. The monitor includes an adjustment for varying the sensitivity of the detection process by changing the magnitude of the predetermined pressure reference signals. The control can also be set to detect a rapid excursion of the measured firing pressure which represents an excessive pressure loss or no pressure signal. Further, when the fluid dispenser is closed, that is, OFF, the same pressure transducer is monitored to detect a pump malfunction. In any of the above situations, the error signal produced is effective to terminate the operation of the fluid dispenser.
The pressure transducer typically used in the analog monitor control described above produces a low level output signal. However, the transducer is located in an environment with the potential for high levels of electrical noise; and therefore, a preamplifier must be located within several feet of the pressure measuring transducer which is attached to the fluid dispenser. In addition, as with most analog systems, the monitor control is susceptible to noise and has a tendency to drift which makes calibration difficult and subject to inadvertent change. Further, in order to obtain a more reliable detection of poorly coated cans, the monitor must detect an unsatisfactory firing pressure over at least two fluid dispensing cycles before a coating error signal is produced. Consequently monitoring the quality of the fluid dispensing cycle on a cycle by cycle, that is, can by can basis, is not available.
A fluid dispensing monitoring system that overcomes some of the disadvantages of the above system is disclosed in Japanese publication No. 61-278373(A) which is assigned to a subsidiary of the assignee of the present invention. With that monitor, a processing unit samples a pressure signal from the fluid dispenser a predetermined number of times while the fluid is being dispensed. Each sampled pressure signal is compared to upper and lower limits of an acceptable pressure range. Further, each of the sampled pressure signals that exceed the upper and lower limits of the acceptable pressure range are individually counted. The control system requires that a predetermined number of sample pressure signals exceed either of the upper or lower limits before an alarm is given. Further, the above sampling process can be used to sample the current and voltage of the solenoid for the flow control valve which is used to open and close the fluid dispenser thereby providing an indication of whether the flow control valve is operating properly.
While the above sampling monitoring system has advantages over the prior analog monitoring control system, it continues to share many disadvantages of prior monitoring control systems for fluid dispensers. While prior controls detect alarm conditions requiring corrective action, the prior controls do not provide a comprehensive methodology of collecting data to provide warning information regarding a pending potential malfunction and what the source of the malfunction may be. Further, prior control systems require that production line operators monitor each individual fluid dispenser at its physical location; and there is no capability of monitoring the status of one or more of the monitor controls at a remote location. Further, with prior systems, each fluid dispenser on the production line has its own monitor control; and while each control system is connected to other process control devices, such as, alarm lights and other indicators, there is little or no detailed information provided to the production line operator with regard to identifying a particular malfunction or the diagnosis of a malfunction. In addition, the prior pressure monitor systems have calibration systems that are relatively difficult to use or can be calibrated to a poor performance, for example, calibrated to a worn nozzle without any indication of a problem.